1. Field of the Invention
The invention relates to modified polyesters which are derived from terephthalic acid or their polyester-forming derivatives and aliphatic diols, preferably those having 3 to 8 carbon atoms, particularly polytetramethylene terephthalate.
2. Discussion of the Prior Art
The invention relates furthermore to a method of preparing modified thermoplastic polyesters, and to molding compositions containing such polyesters.
Thermoplastic molding compositions polyalkylene terephthalates are used in the production of high-quality moldings by the various thermoplastic forming methods, such as injection molding, extrusion and the like. Polytetramethylene terephthalate (PTMT) has acquired special importance. The crystallization characteristics of this product make it possible to fabricate it at desirable low temperatures and economical high output rates.
PTMT is prepared, as a rule, by known methods of polycondensation. In the conventional processes of preparation, except for transesterification catalysts, generally no other additives are put in, so that the polymeric end product contains only residues of these catalysts. The thermoplastic polyesters thus made can be fabricated by the conventional methods, preferably in injection molding and extruding machines. During the fabrication, the molten molding composition is exposed to a more or less severe thermal oxidation due to heat and the presence of oxygen, which expresses itself in a loss of molecular weight and the concomitant loss in the overall quality of the finished articles.
These losses have an especially detrimental effect on the characteristics most important in construction material, namely toughness and rigidity. It is a known measure to add to polyalkylene terephthalates of high molecular weight stabilizing compounds and/or toughness-improving, polymeric, thermoplastic additives, either during fabrication or in a separate incorporating process preceding fabrication.
It is also known to add isocyanates to polyethylene terephthalate (PETP) of high molecular weight. The isocyanates produce a stabilizing action, so that the molecular degradation produced by thermal stress during the fabrication of the high-molecular PETP into injection molded articles is compensated, preserving the properties of the pure PETP (cf. DL Pat. No. 78,381).
These procedures, however, are not satisfactory in actual practice. Improved toughness, for example, is achieved at the expense of an undesirable loss of rigidity. The stabilizers generally have no direct effect on the mechanical characteristics, since all they do is reduce the molecular degradation of the polyesters during the incorporation of the additives and during fabrication.
It is indeed possible by increasing the molecular weight in the preparation of the thermoplastic polyesters to raise the level of the characteristics of the products made therefrom, but their melt viscosity is also increased, and a higher mass temperature is required in the compounding and fabrication, so that the greater amount of work which must be performed in plasticizing them and the longer cycle time combine to place limits on the economy of manufacture of molded products.
In addition, the preparation of polyesters of high molecular weight is very expensive. It is known that, in the preparation of PTMT, mild polycondensation conditions must be maintained, since otherwise thermal damage is done to the polyesters even during the polycondensation, which manifests itself in impairment of the mechanical properties of the products made from them.
It has therefore been proposed to interrupt the polycondensation in the melt at low reduced viscosities of, for example, 0.9 dl/g, and to cure these polycondensates in the solid phase in an additional process step so as to achieve the desired higher reduced viscosity (cf., e.g., Kunststoffe 66 (1976) 10, p. 611, col. 1, par. 2 sqq.). For this purpose, however, reaction times amounting to hours are required, which makes the preparation of polyesters of high molecular weight, especially PTMT, very uneconomical.